Shell and tube heat exchangers have been commonly used as the evaporator component for industrial air conditioning and refrigeration systems. A common usage of such systems is for chilling ocean water for storage and preservation of fish.
In such a system, in a vapor compression cycle, a liquified refrigerant is metered by a thermal expansion valve into the lower pressure environment of the heat exchanger. In the heat exchanger, the refrigerant changes phases from a liquid to a vapor as it absorbs the required heat from the liquid to be cooled. A compressor withdraws the refrigerant vapor from the heat exchanger, raises its pressure and discharges the refrigerant into the condenser, where the heat absorbed in the evaporator is discarded to the heat sink as the refrigerant changes phase from a vapor to a liquid. The higher pressure liquid is then ready for another cycle.
A common type of heat exchanger is a shell and tube heat exchanger which includes a shell having a plurality of tubes disposed therein. Refrigerant flows through the tubes while the fluid to be cooled flows through the shell, externally to the tubes and in contact therewith. As the refrigerant passes through the tubes it evaporates, absorbing heat from the fluid being cooled.
Laminar fluid flow through and along the exterior surface of the tubes is undesirable as it creates an insulating boundary layer contacting the tubes, thereby decreasing the amount of heat that is transferred from the fluid to the refrigerant. To overcome this problem, conventional tube and shell heat exchangers have utilized baffles spaced along the length of the shell and oriented so as to direct the fluid flow substantially along a sinusoidal path such that the fluid flows across the tubes thereby creating turbulence. For instance, U.S. Pat. No. 4,699,211, issued to Geary et al., and U.S. Pat. No. 4,118,944, issued to Lord et. al., disclose a shell and tube heat exchanger having a plurality of baffles spaced along the length of the shell for the purpose of creating a wave-like flow therethrough.
The problem associated with these conventional baffled shell and tube heat exchangers is that the velocity of the fluid passing through the heat exchanger is variable, increasing as it passes by the baffles and decreasing between the baffles. Since heat exchangers are designed for specific flow rates, the changing flow rate associated with a baffled heat exchanger makes the design less efficient. In particular, a baffled heat exchanger must be designed to accommodate the higher flow rates associated with the fluid passing by the baffles. Therefore, the flow rate of the fluid is often below the optimum flow rate for which the heat exchanger is designed. Moreover, the baffle design requires that the tubing be spaced relatively far apart in order to allow adequate fluid flow by the baffles. Another design for accommodating fluid flow by the baffles is to not include tubing proximate the baffles, resulting in portions of the heat exchanger having no tubing, and, therefore, no heat exchange. Each of these designs results in a relatively large heat exchanger.
A further disadvantage associated with a baffled heat exchanger is that the fluid flow in the areas adjacent the baffles is relatively stagnant. Therefore, when ocean water or other contaminated fluid is being cooled, these portions of the heat exchanger become contaminated, thereby reducing the efficiency of the baffled heat exchanger.